a. Field of Invention
This invention pertains to an implantable pacemaker and more particularly to a pacemaker constructed and arranged to provide therapy for atrial fibrillation. More particularly, this invention pertains to a pacemaker having means for providing atrial defibrillation using defibrillation pulses applied precisely in synchronism with ventricular activity. Moreover the invention also pertains to an implantable atrial defibrillator which monitors the ventricle and stabilizes the same if necessary, before the application of atrial defibrillation pulses.
b. Description of the Prior Art
Since ventricular tachyarrhythmias can have grave and potentially lethal consequences, the primary concern for clinicians and pacemaker manufacturers until recently has been the detection and reversion of this undesirable condition. While many pacemakers were also capable of detecting atrial fibrillation, this latter condition was believed to be relatively benign, if unpleasant to the patient. Accordingly, atrial fibrillation was originally either ignored, or treated in a palliative manner.
However, since present pacemakers are capable of handling ventricular arrhythmia, attention has turned toward the detection and treatment of atrial fibrillation. There are several reasons why it is thought to be important to control atrial fibrillation. First, atrial fibrillation is associated with the loss of AV synchrony and therefore can result in a fast and irregular heart rate causing hemodynamically inefficient cardiac operation. Atrial fibrillation has also been associated with other symptoms such as dyspnea, fatigue, vertigo, angina pectoris, and palpitations. These conditions may lead to a patient losing his ability to enjoy life, perform work, and function normally. Atrial fibrillation is also a significant cause of stroke and death, via emboli arising from the left atrium.
Recent studies have shown in fact that atrial fibrillation occurs in 0.3-0.4% of the general population. Moreover, the incidence of atrial fibrillation increases with age, so that it reaches 2-4% by the age 60, and 8-11% for patients above 75 years old.
Drug therapy and/or in-hospital cardioversion are the current clinical therapies provided for atrial defibrillation. However, these approaches are of limited efficacy. Hence, implantable atrial defibrillators (IADs) have been proposed as one of several non-pharmacologic therapeutic options. See for instance U.S. Pat. No. 5,207,219. Studies have shown that internal atrial defibrillation is feasible at low energy levels. This feature is important for IADs because if the defibrillation pulses are successful at low energy, they can be relatively painless and therefore desirable for treating a conscious patient with atrial fibrillation. However one problem with the IADs proposed so far is that it can be difficult to synchronize their defibrillation pulses to the intrinsic ventricular events, because the ventricular rate can be highly variable. If atrial defibrillation pulses are not carefully synchronized with the ventricular intrinsic events, then a ventricular tachyarrhythmia (e.g., ventricular fibrillation) could be induced. Thus, rather then helping the patient, the present IADs may have a negative effect, namely inducement of ventricular arrhythmia. IADs are desirable because they may reduce the number and duration of AF episodes, may improve the lifestyle of the patient, may reduce the number of hospitalizations for treatment of recurring atrial fibrillation and may reduce morbidity and mortality. However, in a device without ventricular defibrillation capability, any instance of ventricular arrhythmia inducement by the IAD is unacceptable. IADs may be designed, in fact, as a dual chamber defibrillator with defibrillation capability in the atrium and ventricle. Even so, it is desirable to greatly minimize the induction of ventricular tachyarrhythmias by the devices.